High-density urban areas often use an underground network grid to distribute electrical power. In this situation, a grid of transformers convert high-voltage (13 kV or greater) feeds from substations into low voltage (under 600V, typically 120V) secondaries. These secondary conductors are connected in parallel, providing a redundant link. The secondary conductors are often 500 MCM copper, with rubber/neoprene insulation. A typical vault or service box would contain a three phase service, and each phase would consist of two parallel 500 MCM conductors from a network transformer. Usually there are two or more sets of these, also connected in parallel. Thus, there may be 6, 12, or 18 500 MCM copper wires, with 2, 4, or 6 wires for each phase. The neutral is also paralleled, but sometimes with not as many conductors.
Although the network grid system is very reliable, periodic maintenance is required, and equipment breakdowns do occur. It is costly and time-consuming to enter an underground vault, due to safety issues as well as logistical issues (e.g. blocking traffic, unwelding manhole covers, etc.) Consequently, the electrical utility often cannot economically monitor the state of the network system.
One failure type is a cable fault. When a cable fault occurs, extremely high currents flow, and this often causes severe damage to the cable, and even to nearby cables. To prevent this, cable limiters are used. These are fast-acting fuses that are designed to open before the cable insulation itself is damaged. They are not designed for overload protection, just fault current protection. These are typically placed in series with each conductor, at every junction and access point in the secondary grid. Doing that minimizes the repair work needed after a fault, and limits the damage to the single faulty cable.
The redundant nature of the grid insures that if a single cable fails, the limiter removes it from the circuit, and the rest of the cables absorb the load. Gradually the capacity of the secondary network is degraded as more faults occur over time. Since the network continues to function, and underground cable inspection is very costly, the utility has no easy way to determine how quickly this is happening, or where the faulty cables are in the network.